In a data frame have a plurality of subframes, it is known that the plurality of subframes can include multiple downlink subframes and multiple uplink subframes. In certain configurations there are fewer uplink subframes than are configured to include acknowledgment/non-acknowledgment messages (ACK/NACK) for the multiple downlink frames.
There is a coverage issue when multiple ACK/NACK bits are transmitted. User equipment in coverage-limited situations may schedule data reception using multiple subframes. Thus, ACK/NACK bundling has been proposed as a way to transmit a single acknowledgment. This can significantly increase the coverage of the physical uplink control channel (PUCCH), and user equipments that are in poor coverage may be configured to operate in this mode. In this mode, a missed scheduling assignment can be problematic if an ACK is transmitted. To minimize this error case, the eNode B can either limit the number of downlink subframes it can schedule to a user equipment with this feedback mode or it an adjust the physical downlink control channel (PDCCH) transmission power to minimize PDCCH error.
For user equipment that is in good coverage areas, there should be no issue in transmitted multiple acknowledgments in order to maximize system throughput. Nonetheless, it still needs to be determined whether user equipment will be configured to feedback a certain number of bits based on the configuration of downlink and uplink or with the user equipment have the flexibility to feedback based on actual number of downlink subframes received. There is the possibility of mismatch between what the user equipment is sending versus what the eNode B may be expecting due to missed scheduling assignments. Coverage can be affected as only one configuration will be supported in a cell.
In view of the foregoing, there is a need to determine how to transmit uplink ACK/NACK for Long Term Evolution wireless communication systems using time division duplex (TDD). In TDD one uplink subframe ACK/NACK from multiple downlink subframes should be transmitted. There are two known modes: single ACK/NACK, which can use ACK/NACK bundling, or multiple ACK/NACK. The question remains how to support both modes, how to transmit multiple ACK/NACK and how to implicitly ACK/NACK resource indication.
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